In the aftermath of the attacks of September 11 and the anthrax scare, we have a heightened awareness of US vulnerability to bioterrorism. One of the most feared infectious agents is variola virus, the causative agent of smallpox. Various strains of vaccinia virus (VV) are highly effective in preventing this disease, but have definite rates of complications. Severe illness or death is rare in people with normal immune responses, but considerably more common in individuals with cell-mediated immune defects. The number of individuals that are at risk from this normally innocuous vaccine has greatly increased with the spread of the human immunodeficiency virus (HIV), and it now becomes important to improve the efficacy and safety of this vaccine. We have worked extensively with VV as a recombinant vaccine for a number of diseases; our rinderpest vaccine was described as one of two outstanding rVVs in a leading journal (G. Ada, Nature 349:369, 1991). We have also developed strategies for attenuating VV while enhancing efficacy, with one of the most effective being the incorporation of the interferon-gamma (IFN-gamma) gene. We have shown that expression of IFN-gamma leads to a TH1 immune response essential against viral infection with no deleterious effects. We have also studied the effects of inactivating VV immunomodulating genes such as B8R, B13R, and B22R that are virulence factors in VV. Based on our past experience, we propose developing a safer and more efficacious vaccine for smallpox based on the New York City Board of Health (Wyeth) strain of VV that is currently used in the US. We will delete the B8R gene and insertionally inactivate the TK virulence gene with the human IFN-gamma gene to increase attenuation of the virus and the protective cell-mediated immune responses. This recombinant VV will be compared to the parental vaccine in both normal and simian immunodeficiency virus-infected macaques (used as a model for HIV-infected humans) to assess efficacy and safety for normal and immunodeficient individuals.